The ionizable fixed charge groups of polyelectrolyte membranes give rise to important electromechanical and swelling phenomena. These membranes can undergo dramatic changes in bulk dimensions and microstructure by reversibly altering the electrostatic swelling forces arising from polyelectrolyte fixed charge groups, thereby changing transport properties. Further, the presence of membrane fixed charge groups give rise to electrokinetic transport phenomena and electrostatic partitioning of charged solutes.
pH control of the ionization state of crosslinked polymethacrylic acid (PMAA) and polyacrylic acid gels can produce reversible swelling changes. pH-induced anisotropic length changes in oriented polyacrylic acid filaments can be used to raise and lower mechanical loads. The extent of swelling is limited by the degree of membrane crosslinking.
The process of gel extraction uses pH-sensitive gels to separate and concentrate solutes by selective partitioning of the solutes within the gels, rather than selective permeability. In this process, as the gel ionizes, it swells, imbibing water and only those solutes accessible within the gel. The gel is removed and the remaining solution contains an elevated concentration of those solutes excluded from the gel. The partitioning of solutes in these gels is selective on the basis of solute, size, and charge (for example, by Donnan partitioning effects). The extent to which changes in gel swelling affect solute permeability and separation is dependent on membrane composition and crosslink density.
One means by which an applied electric field can alter membrane permeability is direct modification of membrane microstructure. Application of a transmembrane electric field induces phase changes in liquid crystal membranes. Using deformable polyelectrolyte membranes, electric field control of intramembrane pH or ionic strength can regulate changes in membrane hydration, and concomittant changes in membrane permeability. Electrodiffusion mediated changes are known to occur in the bulk hydration of PMAA membranes.
The effect of an electric field on the solute reduces concentration polarization and improves membrane filtration. An applied electric field increases filtration flux by electrophoresis of a retained solute away from a membrane filter, and by electroosmosis of the solvent through the filter and adjacent film. Similar permeability changes can be induced without electric fields by direct manipulation of bath pH and ionic strength to modulate membrane swelling forces.
An additional mechanism for electrical control of solute flux are electrode reactions which induce changes in bath pH. Weiss and co-workers measured 16-fold changes in the permeability of PMAA members to Lis-Maltoheptaose after passing current through bare metal electrodes to electrochemically alter the bath PH. Weiss, A. M., et al., AIChE Symp. Ser., 82:88-98 (1986).
Eisenberg and Grodzinsky demonstrated in collagen membranes exposed to a gradient in either pH or ionic strength, changes of up to 25% in permeability to sucrose by applying a transmembrane electric field. Eisenberg, S. R. and Grodzinsky, A. J., J. Membrane Sci., 19:173-194 (1984).
The biotechnology industry provides an increasing demand for new methods to purify proteins from other cell products and impurites. Suitable techniques must be selective, capable of functioning on a large scale, and must avoid denaturation of proteins. Currently, chromatographic techniques are widely used for protein purification but these are both time consuming and labor intensive. Electrophoresis is well suited to small scale analytical separation, but is generally not practical for large scale purification. Ultrafiltration is a cost effective method used for protein concentration, but it lacks the selectivity needed for purification and has the additional problem of concentration polarization, in which solutes retained at the membrane surface form a compact layer that impedes solvent flux.